1. Field of the Invention
The present invention relates to a ferroelectric film, a process for producing the ferroelectric film, a ferroelectric device using the ferroelectric film, and a liquid discharge device using the ferroelectric film.
2. Description of the Related Art
Currently, the piezoelectric devices constituted by a piezoelectric body and electrodes are used, for example, as actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts in correspondence with increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body. The perovskite oxides such as PZT (lead titanate zirconate) are widely used as materials for the piezoelectric body. The perovskite oxides such as PZT are ferroelectric materials having spontaneous polarization (i.e., exhibiting polarization even when no electric field is applied).
Since the 1960s, it has been known that the PZTs doped with various donor ions having higher valences than the atoms substituted by the donor ions are superior to the intrinsic PZT in characteristics including the piezoelectric performance. Various lanthanide cations such as Bi3+ and La3+ ions are known as A-site donor ions with which the Pb2+ ions in the A-sites can be substituted, and the ions of V5+, Nb5+, Ta5+, Sb5+, Mo6+, W6+, and the like are known as B-site donor ions with which the Zr4+ and/or Ti4+ ions in the B-sites can be substituted.
Formerly, the ferroelectric bodies were manufactured by mixing a plurality of types of oxide powder containing constituent elements of a desired material, and molding and baking the mixed powder; or by dispersing in an organic binder a plurality of types of oxide powder containing constituent elements of a desired material, applying to a substrate the organic binder in which the mixed powder is dispersed, and baking the substrate. That is, the ferroelectric bodies were manufactured through a baking process at the temperature of 600° C. or higher (normally at the temperature of 1000° C. or higher). Since the ferroelectric bodies were manufactured through a high-temperature thermal equilibrium state, high-concentration doping with a dopant (which has a valence essentially different from the valence of the material to be doped) is impossible.
S. Takahashi, “Effects of Impurity Doping in Lead Zirconate-Titanate Ceramics,” Ferroelectrics, Vol. 41, pp. 143-156, 1982 reports results of studies on the doping of bulk ceramic PZT with various donor ions. FIG. 33 is a quote from “FIG. 14” in the Takahashi reference, and shows a relationship between the amount of the dopant and the dielectric constant. FIG. 33 shows that the performance is optimized when the amount of the dopant is approximately 1.0 mol % (which corresponds to approximately 0.5 weight percent in FIG. 33), and the performance deteriorates when the amount of the dopant exceeds the optimum amount.
Similarly, Japanese Unexamined Patent Publication No. 2003-055045 discloses that in the case where PZT is doped with niobium (Nb), antimony (Sb), or tungsten (W), the performance of the doped PZT increases until the amount of the dopant is increased to 3 mol %, and decreases when the amount of the dopant exceeds 3 mol %. In addition, Japanese Unexamined Patent Publication No. 2005-035843 discloses that the baking temperature can be lowered by doping PZT with vanadium (V), and the upper limit of the amount of the dopant is 0.4 weight percent. Further, U.S. Pat. No. 6,916,754 discloses that the baking temperature in liquid-phase sintering can be lowered by doping PZT with tungsten (W) and/or molybdenum (Mo), and the upper limit of the total amount of the dopant is 9.8 mol %. Furthermore, Japanese Unexamined Patent Publication No. 7 (1995)-330425 discloses that the variations in the performance can be suppressed by doping PZT with antimony (Sb), and the upper limit of the amount of the dopant is 3.0 weight percent.
As mentioned above, in the case where the ferroelectric bodies are manufactured by the conventional techniques, the upper limit of the amount of the B-site donor ions has been considered to be 9.8 mol %. In addition, when PZT is doped with the upper limit, 9.8 moi %, of the dopant for lowering the baking temperature, other characteristics are sacrificed for the doping.
Moreover, as indicated in T. Tanaka, et al., “Atsuden Ceramics Zairyo (Piezoelectric Ceramics Materials),” in Japanese, Gakkensha, Tokyo, pp. 110-131, 1973, it is known that the performance (of the relaxer ferroelectric materials) is improved by codoping with the donor ions having a higher valence and acceptor ions (such as Ni2+ or Co2+ ions) having a lower valence for balancing the valences. An example of such relaxer ferroelectric materials is PZT doped with Pb(Ni1/3Nb2/3))O3. In this example, PZT is codoped with the Nb5+ ions and Ni2+ ions, instead of being doped with the Nb5+ ions only, so that the average valence of the B-site atoms in the thermal equilibrium state becomes +4. Therefore, doping of PZT with high-concentration niobium is enabled.
However, as indicated in the Takahashi reference, the doping with acceptor ions such as Ni2+ or Co2+ ions decreases the ferroelectric performance. Therefore, it is impossible to sufficiently realize the effect of the doping with the donor ions in the relaxer ferroelectric materials in which the doping with the acceptor ions is performed.
Japanese Unexamined Patent Publication No. 2005-072474, U.S. Pat. No. 7,187,025, Japanese Patent No. 3791614, Japanese Unexamined Patent Publications Nos. 2005-101512, 2006-182642, and 2006-188427, U.S. Pat. No. 7,196,457, and U.S. Patent Application Publication No. 20050236654, which are hereinafter collectively referred to as JP 2005-072474 to US 20050236654, disclose a ferroelectric film which is doped with 10 to 50 mol % of B-site donor ions such as V, Nb, or Ta ions, instead of being codoped with the acceptor ions.
All the ferroelectric films disclosed in JP 2005-072474 to US 20050236654 are formed by using the sol-gel technique. Since the sol-gel technique uses a thermal equilibrium process, when the ferroelectric films are produced in the manners disclosed in JP 2005-072474 to US 20050236654, it is necessary to add silicon (Si) as a sintering assistant in order to promote sintering and realize a thermal equilibrium state. In addition to Si, germanium (Ge) and tin (Sn) are known as a sintering assistant. However, since the doping with such a sintering assistant decreases the ferroelectric performance, when the ferroelectric films are produced in the manners disclosed in JP 2005-072474 to US 20050236654, it is impossible to sufficiently realize the effect of the doping with the donor ions.
The baking temperature can be relatively lowered by adding Si as a sintering assistant when the ferroelectric films are produced in the manners disclosed in JP 2005-072474 to US 20050236654. However, in this case, it is impossible to form ferroelectric films having a thickness greater than 1 micrometer, since cracks are likely to be produced when the thicknesses of the ferroelectric films are increased. Therefore, actually, the greatest thicknesses of the ferroelectric films disclosed in JP 2005-072474 to US 20050236654 are as small as approximately 0.2 micrometers. Such thin ferroelectric films can be used in ferroelectric memories. However, such thin ferroelectric films cannot achieve sufficient displacement in piezoelectric devices. It is preferable that the ferroelectric films in piezoelectric devices have a thickness of 3 micrometers or greater. Although the thickness of the ferroelectric films may be increased by repeating formation of a thin film, the repeated formation for increasing the thickness is impractical.
Further, as indicated in Japanese Unexamined Patent Publication No. 2005-150694, the Ph defect (loss of lead) is likely to occur when the sol-gel technique as disclosed in JP 2005-072474 to US 20050236654 is used. There is a tendency that the ferroelectric performance decreases when the Pb defect occurs. Therefore, use of the sol-gel technique as disclosed in JP 2005-072474 to US 20050236654 is not desirable.